juju_machine_agent_name can return an invalid machine name.

[Pedro Castillo]

The `juju_machine_agent_name` from the juju-introspect scripts can return an invalid machine name. The script will match any directory under `/var/lib/juju/agents` whose name starts with `machine`, which is generally only the `machine-N` directory, but in some edge cases it will grab other directories and concatenate the values into an invalid machine tag.

Here is the bash function which grabs the machine name:
```
juju_machine_agent_name () {
  local machine=$(find /var/lib/juju/agents -type d -name 'machine*' -printf %f)
  echo $machine
}
```

Here is an edge case where the script grabs an invalid machine name:
```
ubuntu@juju-7cfc1d-18-lxd-10:/var/lib/juju/agents$ juju_machine_agent_name
machinelearningmachine-18-lxd-10
```

Here is the same `find` command but echoing the full path of the directories it matched:
```
ubuntu@juju-7cfc1d-18-lxd-10:/var/lib/juju/agents$ find /var/lib/juju/agents -type d -name 'machine*'
/var/lib/juju/agents/unit-gnocchi-3/.venv/lib/python3.6/site-packages/botocore/data/machinelearning
/var/lib/juju/agents/machine-18-lxd-10
```

This was found on an LXD container on an environment running juju-agent version 2.9.18.

I believe that modifying the `juju_machine_agent_name` to replace the `find` command with `ls /var/lib/juju/agents | grep '^machine-' would be a good fix. Otherwise, adding a `-maxdepth 1` to the `find` command would also fix the issue.

[John A Meinel (jameinel)]

This should be a very simple fix, and I agree that we shouldn't be recursing into any sub directories.

[Caner Derici (cderici)]

https://github.com/juju/juju/pull/14888 should fix this.